This invention relates to percussion drilling tools, sometimes referred to as down-hole-percussion drill motors, which are used for oil drilling and penetration of rock substrate, often to great depths. In such tools a motor casing is generally provided for housing a reciprocating piston member for hammering an anvil and bit shank piece by means of regulated air pressure.
In conventional drills of this type, such as shown in U.S. Pat. No. 3,503,459, the air pressure is released from the drill bit and forced together with the cuttings and any other debris including water up the annular space between the drill stem and the hole. In this way the bottom of the hole is continually flushed and kept relatively clean to enable it to be broken up by the pneumatic action of the reciprocating bit. In many instances, however, when increasing pressures are encountered at great depths in the form of subterranian water, a "flood out" condition will occur in which the water pressure at the bottom of the hole will be equal to or greater than the air pressure applied to the pneumatic hammer so that further progress is severly impeded if not curtailed due to the lack of pressure differential. When this occurs, the drill stem is usually tripped out of the hole and the pneumatic hammer and percussion bit are replaced with a rotatable tricone bit assembly in which the primary mode of penetration is rotary rather than reciprocal. This mode is less efficient and slower and requires a drill mud as a flushing agent for the cuttings, the mud being introduced usually through the drill stem and forced out the drill bit and up the annular spacing between the drill stem and the hole. It should be apparent that tripping out of the hole and replacement of the bit requires a costly delay and effort in the drilling operation which most drill riggers try to avoid.